Much of pulmonary physiology is based on the analysis of respiratory gases and the mass spectrometer has shown its usefulness as a high speed accurate gas analyzer. The mass spectrometer is an apparatus that separates charged particles (ions) according to their mass-to-charge ratios and determines the relative abundance of each type of ion present.
Mass spectrometers used in pulmonary applications generally include a sample-inlet assembly, an ionization chamber, a focusing lens, a mass filter in a filter chamber and a sensor, all housed in a low pressure vacuum envelope. Examples of such prior mass spectrometers are found in U.S. Pat. Nos. 4,008,388, issued to McLafferty et al., and 4,816,685, issued to Lange. The sample-inlet system captures the respiratory gas to be analyzed and directs it to the ionization chamber. A stream of electrons from a filament bombards the gas entering the ionization chamber and causes the gas molecules to lose electrons thereby producing positive ions. The ions alone are focused into a beam and accelerated into the filter chamber. The electrons are not allowed to pass into the filter chamber. In the filter chamber, the ion beam is sorted into its components on a mass-to-charge ratio by the mass filter.
In the filter chamber, a mass filter, as for example a quadrupole mass filter, is utilized to separate ions by their mass-to-charge ratios. The filter does so by the application of an electric and/or magnetic field. The filter is designed such that the ions of the molecule to be measured, for example preselected oxygen (M/e 32) ions, continue through the filter chamber and are collected and measured by the sensor. The remainder of the ions, for example the non-oxygen ions, remain in the filter and do not migrate to the sensor.
It is these remainder, or unselected, ions that do not migrate to the sensor that cause problems with the stability and sensitivity of the mass spectrometer. Under normal operation, these unselected ions contact the filter elements, pick up electrons from those elements to become neutralized, and eventually migrate from the filter and are removed from the filter chamber by the ion pump. However, continued operation can result in a build up of these unselected ions on the filter elements that in time creates a dielectric film which prevents the ions from picking up electrons from the elements. That film will eventually take on a charge of its own and interfere with operation of the filter, its stability and sensitivity. In the past, to retain its sensitivity and the stability of the mass spectrometer, the practice was to disassemble it and mechanically or chemically clean the filter elements to remove that film. This disassembly has the obvious disadvantage in that it causes down time of the mass spectrometer and additional expense along with reduced sensitivity and stability of the mass spectrometer before the disassembly.